Relative Compression vs Injector Contribution Differentiation
Always compare injector contribution results with relative compression results before deciding between injector replacement and mechanical engine work.
Fault Overview
Process page for differentiating low injector contribution (fueling) from low relative compression (mechanical) when a cylinder shows weakness on balance testing.
Engine & Platform Coverage
Symptoms
- •Relative compression vs contribution
- •Low compression or injector
- •Weak cylinder differentiation
- •Mechanical vs fueling imbalance
Possible Causes
- 1.N/A — differentiation process. Low contribution with normal compression points to injector or fueling; low compression with normal contribution points to mechanical (rings, valves, etc.).
Wiring & Circuit Notes
Electrical injector circuit proof remains the first step before either test is fully trusted.
DVOM Specs & Sensor Tests
Run both OEM relative compression and contribution/balance tests under specified conditions. Compare results cylinder by cylinder. Supplement with return-flow if fueling is suspected.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Step 1 — Clear Electrical Injector Faults First: Clear electrical injector faults first. Electrical open/short codes must be resolved before either test is fully trusted.
- 2Step 2 — Run Injector Contribution/Balance Test: Run injector contribution/balance test and record results for all cylinders.
- 3Step 3 — Run Relative Compression Test: Run relative compression test and record results for all cylinders.
- 4Step 4 — Compare the Two Data Sets: Compare the two data sets cylinder by cylinder. Technicians sometimes replace injectors on a cylinder that actually has a mechanical compression issue.
- 5Step 5 — Low Contribution, Normal Compression → Fueling Path: If contribution is low and compression normal → proceed to return-flow and injector diagnosis.
- 6Step 6 — Low Compression, Normal Contribution → Mechanical Path: If compression is low and contribution normal or secondary → proceed to mechanical diagnosis (leak-down, etc.).
- 7Step 7 — Both Low → Address Mechanical First: If both are low → address mechanical first, then recheck fueling after mechanical repair.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Injector service if fueling side confirmed $300–700 per injector; mechanical engine repair if compression side confirmed — variable. Avoid mixing the two repair paths.
Start Diagnosing This Fault Now
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
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AI Diesel Tech — The Technician's Diagnostic Engine
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Summary
Use the AI Diesel Tech diagnostic engine to run this code through a structured, evidence-gated sequence and get a confirmed root cause before any parts are replaced.